Developing Predictive Models For The Kinetics Of Heavy Metal Adsorption And Desorption Reactions In Soils

Soil sustains human life,which governs many important processes and reactions.However,with the rapid development of economy and industry,soil heavy metal pollution has become one of the most severe environmental problems.To ensure human health and achieve sustainable development,it is urgent to control the soil pollution by heavy metals.Soil organic matter,metal(hydr)oxides,and clay minerals are key soil components and metal sorbents.Heavy metal reactions with soil components are mainly through metal binding with different soil binding sites.Due to the site heterogeneity,metal reactions with different binding sites may have different reaction rates,which may significantly affect the speciation,fate and transport of heavy metals in soil.Therefore,to assess heavy metal reactivity and bioavailability in soil,it is essential to specifically consider chemical reactions between heavy metals and different soil components/binding sites.In soil,the most common chemical kinetic reactions include adsorption,desorption,precipitation,dissolution,etc.,which may control the dynamic speciation of heavy metals at different time scales.In this study,kinetics of heavy metal adsorption/desorption in different soils for five typical heavy metals(Cd,Cu,Ni,Pb and Zn)was studied using a stirred-flow reactor under various reaction conditions.The chemical speciation equilibrium models,Windermere Humic Aqueous Model 7(WHAM 7)and Charge Distribution and Multisite Surface Complexation Model(CD-MUSIC)were integrated into the adsorption/desorption kinetis model.The kinetics model established two key relationships between the thermodynamic equilibrium and kinetic reactions for metal binding to soil binding sites:(1)for one specific site i,we established the relationship between kinetic reaction rate coefficients(adsorption/desorption rate coefficients)and the equilibrium partition coefficients;(2)for different binding sites of the same soil components,we established the relationship between the desorption rate coefficients and metal binding constants of various binding sites.Based on the adsorption/desorption model,this work further developed a multi-reaction models for Zn desorption kinetics based on the experimental results from batch kinetic experiments and X-ry absorption spectroscopy.We specifically elucidated the roles of different Zn species and reaction processes during the kinetic processes.The kinetics models developed in this study have general applicability,which can specifically account for the variations of soil compositions and solution chemistry conditions.The models can also consider the effects of different soil components and binding sites on the kinetic reactions of heavy metals at various time scales.Moreover,the models developed in this study can also analyze the dynamic changes of heavy metal speciation and the distribution of metals among different soil components or binding sites during the reaction processes,which helps to understand roles of different soil components and binding sites accounting for metal kinetics behavior.Our models provide theoretic basis for assessing the fate and risk of heavy metals.The models can be expanded to more complex soil systems based on the soil characteristics and other reaction processes.